The invention relates to a process for the preparation of a material comprising a framework consisting of shell-like structures and an interconnecting pore system, a process for the preparation of a material consisting of shaped bodies connected via webs, a process for the preparation of a material which comprises a framework of shell-like structures and whose interconnecting pore system is filled with shaped bodies connected via webs, materials producible by means of these processes as well as their use as bone replacement material, artificial bone, implant, filter and drug delivery system.
Lost bone, for instance due to a trauma caused by an accident or after the resection of tumors, after infections or by idiopathically developing bone cysts, has been representing a serious problem for surgeons for a long time. It has, for example, been tried to replace bone artificially, to use animal bone, to process animal bone accordingly such that it is not rejected in humans after it has been inserted or also to preserve bone from humans in freezers and to insert it as frozen homologous bone.
There are various methods for processing animal bone for the use in humans. Ivory, for example, has been also used. A bone replacement which is particularly accepted in surgery is the so-called "Kiel bone chip".
All chemically processed heterologous transplants from animals, however, are incorporated into the host bone incompletely or not at all. The bank bone very often leads to lethal infections, i.a. AIDS infections. Fresh homologous bone transplant are affected by such complications to a much greater extent. In this case, it is even possible to transplant tumors. Therefore, it has repeatedly been tried to develop bone replacement materials and methods by means of which the disadvantages of the immune reaction and the transmission of diseases can be avoided.
In DE-A-39 03 695, a process for the preparation of absorbable ceramics on the basis of tricalcium phosphate is described which can be used as a bone replacement material and in which natural bone material from which the soft tissue has been removed is used as a starting material. Residual organic substance is removed from the bone material by pyrolysis an the remaining bone material consisting of hydroxylapatite is subsequently treated with phosphate carriers and then subjected to sintering. This ignited bovine bone material also exhibits good incorporation properties.
The mentioned materials derived from animals, however, cannot fulfil the demands on bone replacement material with respect to reproducibility of the structure, in particular in view of the material strength and the standardizability of the manufacturing process, and additionally their production is very expensive. Therefore, in medicine it is desirable to produce the used materials, for instance bone replacement materials, fully synthetically. These materials should guarantee maximum strength at a minimum of material and should provide a variety of properties for a variety of indications. It is, for example, desirable to provide a process by means of which on the one-hand bone-like trabecular structure , i.e. a "positive" of the bona, and on the other hand a "negative" of the bone, i.e. shaping negative molds of the medullary cavities of the bone can be produced. By means of such a process, on the one hand the physiological structure of bone could be reproduce and, by means of the negative, on the other hand supporting and guiding frameworks could be provided around which bone grows and which are very rapidly able to take up high loads.
In DE-A-40 33 291, a process is described by means of which the aforementioned demands can be achieved in some respects. In this process, firstly preferably spherical shaped bodies are connected to one another to form a three-dimensional conglomerate of shaped bodies, thereafter a material different from the material of the shaped bodies is molded around the shaped bodies in order to form a three-dimensional framework, and subsequently the shaped bodies are removed so that only the three-dimensional framework is left. In subsequent process steps, the cavity system resulting from the removal of the shaped bodies can be again filled wit a ceramic mass, for example, and subsequently the three-dimensional framework can be removed. Thus, by means of this process both a "positive" of the bone and the above described "negative" of the bone can be produced. The preparation of the conglomerate of shaped bodies (negative) serving as the starting material is, however, rather money- and time-consuming and not in any case reproducible
DE-A-22 42 867 discloses a process for the preparation of implantable, porous, ceramic bone replacement, bone composite or prosthesis anchorage materials. In this process, firstly an "auxiliary framework" is prepared of spheres which approximately corresponds to the pores and pore connections of the finished material, and subsequently a castable ceramic mass is filled into the framework. After the ceramic mass has at least partially solidified, the auxiliary framework is decomposed and removed. The process for the preparation of the initial auxiliary framework made of spheres can be, however, at best standardized in a time- and money-consuming manner and is hardly reproducible.